An optical modulating method for applying identical modulating signals having a predetermined phase difference (i.e., time delay) into a plurality of modulating electrodes, such as a SSB (Single-Side band) modulating method, a duobinary modulating method, or the like, has been conventionally utilized in the field of optical communication or optical measurement.
The SSB-type optical modulator typically use an optical waveguide device having a so-called nested waveguide structure with a double nested Mach-Zehnder type optical waveguide where two sub Mach-Zehnder type optical waveguides are incorporated into the branch waveguides of a main Mach-Zehnder type optical waveguide thereof. In such an optical waveguide device, in order to obtain the SSB modulating signals, two split modulating signals are processed to have a phase difference of 90° by a 90° hybrid and are applied to the modulating electrodes of the respective sub Mach-Zehnder type optical waveguides, respectively (see Patent Reference 1).    Patent Reference 1: JP-B2 3867148    Patent Reference 2: JP-A 6-289341
A dual-electrode Mach-Zehnder type optical modulator is used for the duobinary modulating method, whereby the modulating signals respectively applied to the modulating electrodes of the two branch waveguides are configured to have a predetermined bit difference therebetween by using a bit delay circuit. A bit delay is generally selected from the range of 0 to 2 T (T=1 bit).
However, in the optical waveguide device having the nested waveguide structure used for the SSB modulating method, particularly, a bias point, that is, an operating point, of each sub Mach-Zehnder type optical waveguide is frequently shifted under operation, and thus, there is a problem that it is difficult to ensure a stable operation.
In order to suppress such an operating point shift, an output signal from the optical waveguide device is appropriately monitored by a light detecting unit, and when an operating point shift occurs, the shift is appropriately corrected by using a controller or the like. However, the operation for adjustment of the operating point is extremely complex, which makes it difficult to put the optical waveguide device having the nested waveguide structure into practical use.